Fix T89952: GPencil channel box selection offset

The channel box selection was offset for grease pencil layers.

This is a proposed fix by @yann-lty

Before:
{F10227973}

After:
{F10227974}

Reviewed By: #grease_pencil, antoniov

Maniphest Tasks: T89952

Differential Revision: https://developer.blender.org/D11962

source/blender/editors/animation/anim_channels_edit.c

@@ -2683,8 +2683,9 @@ static void box_select_anim_channels(bAnimContext *ac, rcti *rect, short selectm
   /* loop over data, doing box select */
   for (ale = anim_data.first; ale; ale = ale->next) {
     float ymin;
-    /* Skip grease pencil datablock. Only use grease pencil layers. */
+
     if (ale->type == ANIMTYPE_GPDATABLOCK) {
+      ymax -= ACHANNEL_STEP(ac);
       continue;
     }